`(43) THREADED BARREL- The term `threaded barrel' means a feature or characteristic that is designed in such a manner to allow for the attachment of a firearm as defined in section 5845(a) of the National Firearms Act (26 U.S.C. 5845(a)).'.

So what is a firearm in section 5845(a)?

CRC

"Therer are many things I admire about America but one thing I do not is their gun culture." Prime Minister John Howard of Australia